1. Technical Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit structured such that two electrode assemblies are disposed opposite to each other in a central sealed chamber. Each electrode assembly includes an electrode rod, molybdenum foil and a lead wire. One electrode is sealed in a primary pinch seal portion and a second electrode is sealed in a secondary pinch seal portion. The central sealed chamber encloses light emitting substances. The present invention also relates to a method of manufacturing the arc tube.
2. Prior Art
FIG. 5 shows a conventional discharge lamp unit which incorporates an arc tube 5 having a front end supported by one lead support 2 projecting forward from an insulating base 1. A recess 1a of the base 1 supports the rear end of the arc tube 5. A metal support member S, secured to front surface of the insulating base 1 holds a portion of the arc tube adjacent to the rear end of the arc tube. A front lead wire 8, extending from the arc tube 5, is welded to the lead support 2, while a rear lead wire 8 penetrates a bottom wall 1b having the recess 1a of the base 1 formed therein. Then, the rear lead wire 8 is securely welded to a terminal 3 provided in the bottom wall 1b. Symbol G represents an ultraviolet-ray shielding globe arranged to remove an ultraviolet-ray component in the wavelength region harmful to the human body. The ultraviolet-ray shielding globe forms a cylindrical shape and is integrally welded to the arc tube 5.
The arc tube 5 has a sealed chamber portion 5a formed between a pair of front and rear pinch seal portions 5b. The sealed chamber portion 5a has electrode rods 6 disposed opposite to each other and contains light emitting substances. In the pinch seal portions 5b, the sealed molybdenum foil 7 connects the electrode rod 6 projecting into the sealed chamber portion 5a to the lead wire 8 extending from the pinch seal portion 5b to each other. Thus, the pinch seal portions 5b remain airtight.
Preferably, the electrode rod 6 is made of tungsten exhibiting excellent durability. Tungsten has a coefficient of linear expansion that is considerably different from that of quartz glass that constitutes the arc tube. Worse, only unsatisfactory conformability with quartz glass is permitted and the permitted airtightness is unsatisfactory. Therefore, the molybdenum foil 7 having a coefficient of linear expansion similar to that of quartz glass and exhibiting relatively satisfactory conformability is connected to the tungsten electrode rods 6. The pinch seal portion 5b seals the molybdenum foil 7. Thus, the pinch seal portion 5b remains airtight.
A method of manufacturing the arc tube 5 is arranged as shown in FIG. 6(a). An electrode assembly A comprises the electrode rod 6, the molybdenum foil 7 and the lead wire 8. The components are integrally connected. The electrode assembly A is initially inserted into an end of either opening of a cylindrical glass tube W having a spherical expanded portion w2 disposed at an intermediate position of a straight extending portion w1. Then, adjacent position q1 of the spherical expanded portion w2 is first pinch-sealed.
Referring to FIG. 6(b), a light emitting substance P is introduced into the spherical expanded portion w2 through the other end opening of cylindrical glass tube W.
Referring to FIG. 6(c), a second electrode assembly A is inserted. A second pinch sealing operation seals the spherical expanded portion w2, while simultaneously cooling the spherical expanded portion w2 by using liquid nitrogen to prevent both vaporization of the light emitting substance P and heating the adjacent position q2 of the spherical expanded portion w2. The final result is an arc tube 5 having the chipless sealed chamber portion 5a. The first pinch-sealing step shown in FIG. 6(b) uses inactive gas (in general, which is low-cost argon gas or nitrogen gas) as forming gas into the glass tube W in order to prevent oxidation of the electrode assembly A. In the second pinch-sealing step shown in FIG. 6(c), the ends of the openings in cylindrical glass tube W are closed and cooling with liquid nitrogen prevents vaporization of the light emitting substance P. Therefore, a state of near vacuum is necessary for the pinch-sealing operation.
Since a great temperature change occurs when the arc tube is energized, thermal stress is produced between the molybdenum foil 7 and the glass layer. The molybdenum foil 7 and the glass layer each have considerably different coefficients of linear expansion. Recent developments in arc tubes allow instantaneous lighting. Therefore, a high temperature-rise ratio is realized, which rapidly produces thermal stress. Although the molybdenum foil has a relatively satisfactory conformability with glass, repeated cycling of the arc tube causes a gap to be formed between the molybdenum foil 7 and the glass layer in the primary pinch seal portion. Thus, the sealed light emitting substance P leaks into the foregoing gap, thereby changing the characteristics of the arc tube. Thus, a lighting defect occurs and the life of the arc tube is shortened.
In view of the foregoing, an object of the present invention is to provide a method of manufacturing an arc tube for a discharge lamp unit with which satisfactory conformability is realized between molybdenum foil and a glass layer in a primary pinch seal portion and improving the adhesion of the glass layer to the molybdenum foil and an arc tube.
To achieve the foregoing object, according to one aspect of the invention, there is provided a method of manufacturing an arc tube for a discharge lamp unit, comprising a primary pinch-sealing step for mounting an electrode assembly in an open end of a glass tube, the glass tube including a chamber portion, and the electrode assembly comprising an electrode rod, a connecting foil and a lead wire integrally connected in series, the primary pinch-sealing step further comprising inserting the electrode assembly into the open end of the glass tube such that a leading end of the electrode rod projects into the chamber portion, pinch-sealing a first region of the primary pinch seal portion of the glass tube such that a portion of the connecting foil that is connected to the lead wire is contacted by the glass tube, and maintaining a vacuum inside the glass tube under vacuum and pinch-sealing a second region of the primary pinch seal portion of the glass tube such that a portion of the connecting foil and the electrode rod are contacted by the glass tube.
According to another aspect of the invention, there is provided a method of manufacturing an arc tube for a discharge lamp unit, further comprising supplying a substance to the inside portion of the glass through an open end of a glass tube, and a secondary pinch-sealing step for mounting an electrode assembly in the open end of a glass tube, the electrode assembly comprising an electrode rod, a connecting foil and a lead wire integrally connected in series, the secondary pinch-sealing step further comprising inserting the electrode assembly into the open end of the glass tube such that a leading end of the electrode rod projects into the chamber portion, supplying an inactive gas to the inner portion of the glass tube, pinch-sealing a region of the glass tube such that a portion of the lead wire is contacted by the glass tube, cooling the inactive gas in order to liquefy the inactive gas, thereby producing a vacuum in the glass tube, and pinch-sealing a secondary pinch seal portion of the glass tube such that the electrode rod, the connecting foil and the lead wire are contacted by the glass tube.
According to another aspect of the invention, there is provided an arc tube for a discharge lamp unit comprising a glass tube having a sealed chamber disposed at a lengthwise-directional intermediate portion of the glass tube in which a light emitting substance is sealed, the glass tube having a primary pinch seal portion disposed adjacently to the sealed chamber and a secondary pinch seal portion disposed adjacently to the sealed chamber and on an opposite side from the primary pinch seal portion, a pair of electrode assemblies, each comprising an electrode rod, molybdenum foil and a lead wire integrally connected in series, wherein one of the electrode assemblies is enclosed in the primary pinch seal portion such that the lead wire projects from the primary pinch seal portion, wherein the primary pinch seal portion is formed by inserting the electrode assembly into an open end the glass tube such that a leading end of the electrode rod projects into the sealed chamber, pinch-sealing a first region of the glass tube such that a portion of the molybdenum foil that connects the lead wire is contacted by the glass tube, evacuating an inside portion of the glass tube and pinch-sealing the remainder of the glass tube between the first region and the sealed chamber.
According to another aspect of the invention, there is provided an arc tube for a discharge lamp unit, wherein the secondary pinch seal portion encloses one of the pair of electrode assemblies such that the lead wire projects from the secondary pinch seal portion, the secondary pinch seal portion is formed by inserting the electrode assembly into the open end of the glass tube such that a leading end of the electrode rod projects into the sealed chamber, introducing an inactive gas into the sealed chamber, pinch-sealing a portion of the glass tube such that it only contacts the lead wire, cooling and liquefying the inactive gas in order to produce a vacuum in the glass tube and pinch-sealing the secondary pinch seal portion such that the electrode rod, the connecting foil and the lead wire are all contacted by the glass tube.
When a pinch-sealing operation is performed, the pressure exerted by a pincher and the negative pressure in the glass tube act on the glass layer, which is softened due to applied heat. Thus, the glass layer is pressed against the surface of the molybdenum foil so that adhesion without any gap is realized. The portion which has been pinch-sealed during the pinch-sealing step causes satisfactory conformability to be realized between the molybdenum foil and the glass layer. The two elements are firmly joined to each other. Unlike conventional technology, no gap is formed between the glass layer and the molybdenum foil that allows the undesirable leaking of the sealed substance from the chamber portion.
According to another aspect of the invention, there is provided a method of manufacturing an arc tube for a discharge lamp unit wherein the length of the main pinch seal region L2 is not lower than about 50% of the overall length of the temporary pinch seal region L1, and a portion of the main pinch seal region overlaps the temporary pinch seal region.
As the main pinch seal region of the primary pinch seal portion is elongated, the adhesion between the glass layer and the molybdenum foil can be improved. Moreover, the bonding strength between the two elements is increased. Therefore, it is preferable that the main pinch seal region is elongated. Formation of a gap between the glass layer and the molybdenum foil caused from thermal stress must be prevented. Therefore, it is preferable that the length of the main pinch seal region is 50% or longer the overall length of the primary pinch seal portion to make the portion longer than about half of the overall length of the molybdenum foil to be pinch-sealed.
Since the main pinch seal portion overlaps the temporary pinch seal portion in the axial direction, the overall region to be pinch-sealed can reliably be pinch-sealed.
According to another aspect of the invention, wherein a pressure of 400 Torr or lower is maintained in the inside portion of the glass tube when a pinch-sealing step is performed.
When the pressure in the glass tube is not lower than the 400 Torr, the conformability between the glass layer and the molybdenum foil becomes insufficient and a firm joint cannot be realized. When the pressure is 400 Torr or lower, satisfactory conformability can be realized between the glass layer and the molybdenum foil. As a result, firm joint can be realized.